A casting operation with a continuous casting machine is carried out by the steps of: pouring a molten metal into an upper opening of a cylindrical mold having a water-cooled inner wall and upper and lower openings, cooling the molten metal through the wall in contact with the molten metal to provide a cast slab having an outer solidified shell, continuously withdrawing the cast slab from the lower opening of the mold, further cooling the cast slab, and cutting the solidified slab to a predetermined length after the central portion thereof is solidified to provide a product slab to be used as a starting material for rolling, etc.
In carrying out continuous casting, it is necessary not only to improve productivity by preventing defects such as over-pouring and break-out, occurrence of which is undesirable for stable operation, but also to improve quality of product slabs by achieving uniform cooling and solidifying of molten metal within a mold. One means for achieving these goals is to control the level of molten metal so as to keep a predetermined level within a mold.
The level of molten metal is generally controlled using feedback by detecting the molten metal level within a mold, by calculating PID on the basis of deviation between the determined level and the target level so as to determine an operating position of a pouring means such as a sliding gate and a stopper, which are provided in a pouring nozzle to control pouring into the mold, and supplying a control signal to operate an actuator, e.g., an oil hydraulic cylinder for the pouring means to make the deviation zero.
However, in the operation of a continuous casting machine, deposition and removal of precipitates such as alumina onto or from a contact surface between the molten metal and the pouring means, for example, repeatedly occur, and a gain of pouring rate of the molten metal passing through the pouring means varies, resulting in a fluctuation in the molten metal level. In addition, since the inside of the cast slab when it is pulled downward out of the mold is in an unsolidified state, the cast body repeatedly swells and shrinks between pinch rolls. This phenomena is called bulging. As bulging occurs, molten metal within the cast slab is forced up or down, resulting in a level fluctuation of molten metal within the mold.
Thus, there are many disturbances affecting molten metal level control. On the other hand, nowadays, it is greatly desired to increase the casting rate in a continuous casting operation. It is difficult, therefore, to control the molten metal level with great accuracy and a quick response time even by employing feedback control
In the past, as disclosed in Japan Patent Laid-Open Application No. 5-23811/1993, disturbances were successively estimated by using a control signal applied to a molten metal pouring means or a signal indicating the operating position of the pouring means together with a signal indicating the molten metal level within a mold, and correction was made to the control signal so that estimated disturbances were eliminated. Thus, a method of controlling the molten metal level with high accuracy and a quick response time was employed to give intended effects to a certain extent.
On the other hand, sometimes a withdrawing rate of a cast slab out of a mold must be varied during operation of a continuous casting machine depending on the specifications of the product slabs. In addition, in order to produce product slabs having a good internal structure, sometimes the thickness of cast slabs withdrawn out of a mold must be reduced while the slabs have a liquid core, i.e., a liquid core reduction device. Changes in operating conditions in these cases result in a level fluctuation of molten metal within the mold.
In the past, when changes in operating conditions were expected because the withdrawing rate increased or decreased, or the liquid core reduction was carried out, disturbances resulting in a level fluctuation of molten metal within tho mold were estimated, and control for compensating for the variation was previously determined. Feed forward control was carried out to apply additional signals to the control signal for a pouring means at the beginning of changing the operating conditions so as to suppress a transient change of the molten metal level within the mold.
Such a variation in the molten metal level is large and occurs in a rapid change. On the other hand, a system to be finally controlled is a pouring means which is operated in a mechanical manner. The molten metal level moves a marked distance away from a target level before the pouring means finishes its operation in response to a control signal including a signal to compensate for the before-mentioned disturbances. During this transient period after the operating conditions are changed, product slabs degrade with respect to their quality and they must be cut off, resulting in a decrease in the product yield.
These problems can be solved to some extent by employing a control system disclosed in the before-mentioned Japanese Patent Application Laid-Open Specification No. 5-23811, for example, by improving response properties of the control system and by employing a translation actuator, i.e., a stepping cylinder, as an actuator for the pouring means so that the mechanical response of the pouring means can be improved. However, such a solution is not adequate when high speed operation is carried out.
Furthermore, recently, liquid core reduction has been practiced to produce a thin cast slab. According to this process, since a reduction in thickness is applied to a cast slab while the slab has a liquid core, a thin slab can be produced under a relatively mild load. A fluctuation of the molten metal level within the mold, however, is inevitable in this process because molten metal is squeezed from the core portion of the slab when a liquid core reduction is applied to the slab.